"Match accepted" stays for too long and take my time to show my champion

Afte a match is found and I click "accept" the window stays for so long that all my teamates are already in lobby, while I come when there is 1-2s left to show my pref champion. It happen 4 times already and I always come at 1-2s so I guess it's a bug EDIT: I use the same computer in my rented flat and home. I have this problem only at flat so there must be something with internet or router there. I had the same net and router for a year and the problem started few months ago so there must had been something in LoL patch that caused this. So I doubt that reinstalling os or turning protocol IPv6 as Riot suggested will help.
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